Blender Render: PNG image on semi-transparent colored object
I'm not sure to understand what you mean, but I feel you maybe just need to enable this flag in the "shadow" section of the blender internal material settings: "receive transparent".

Here I created a (guess somewhat) similar setup, and without that flag the result is like this



while after enabling it, you can see the "hidden" image behind the semitransparent "glass"



note I here used "raytrace" transparency, but it should work with "Z transparency" too...